5.4 Gravitational Fields Flashcards
What causes gravitational fields?
Any object with mass
How do gravitational field lines represent gravitational fields? (3)
At right angles to object
Arrows show direction of force (always attractive)
Separation of lines shows strength of field
Define gravitational field strength
The gravitational force exerted per unit mass at that point in the gravitational field
How is gravitational field strength and distance related?
Gravitational field strength is inversely proportional to the distance from the centre of mass of the object
What is assumed about the Earth’s gravitational field strength?
It is constant close to the surface of the Earth as the field is considered to be uniform
What does Newton’s law of gravitation assume? (2)
Point masses
Mass is concentrated at centre of mass
Why is there a negative sign in Newton’s law of gravitation?
Gravity is always an attractive force
What is Kepler’s first law of planetary motion?
The orbit of a planet is an ellipse with the sun at one of the two foci
How is a gravitational field shown to be uniform?
Equidistant, parallel field lines
State Newton’s law of gravitation
The force between two point masses is directly proportional to the product of the masses and inversely proportional to the square of their separation
What is eccentricity?
A measure of how elongated a circle is
What is Kepler’s second law of planetary motion?
A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time
What is Kepler’s third law of planetary motion?
The square of the orbital period of a planet is directly proportional to the cube of its average distance from the Sun
How is Kepler’s third law derived? (3)
Model the planetary orbit as a circle
Centripetal force on planet = gravitational force on planet
Determine speed by dividing circumference by orbital period
What is a geostationary satellite?
A satellite that stays above the same area of Earth
What are the conditions for a geostationary satellite? (4)
36000km from Earth’s surface
Orbit above the equator
Time period of 24 hours
Orbit in the same direction as Earth’s rotation
What are polar orbits used for? (2)
Mapping
Reconnaissance
What are Low Earth Orbits used for? (2)
Photography
Weather
What are Medium Earth Orbits used for?
GPS
What are geostationary orbits used for? (2)
Communications
TV
What are High Earth Orbits used for?
Research
Define gravitational potential
The work done per unit mass to move an object to a point in a gravitational field from infinity
What does infinity refer to in the definition of gravitational potential?
A distance so far from the gravitational field that its field strength is zero
What is true about gravitational potential at infinity?
It is zero
Why are all values of gravitational potential negative? (2)
All masses attract each other
Work must be done to separate objects gravitational potential is zero at infinity
What does a graph of gravitational potential against distance look like? (3)
Most negative value at surface of object (graph starts here)
Inversely proportional relationship
All values of gravitational potential are negative
How does gravitational potential change when masses move? (2)
Moving towards a point mass decreases gravitational potential (more negative)
Moving away from a point mass increases gravitational potential (less negative)
What does the area under a force-distance graph for a point mass represent?
Work done
What is escape velocity? (2)
The minimum velocity required to escape the gravitational field of a mass
When loss in KE = gain in GPE
